The network's reach combines the 45 million monthly unique visitors across IAC's e-commerce sites--including Match.com, Evite and Ticketmaster--along with the 10 million monthly visitors across HealthCentral's condition-specific sites.

HealthCentral CEO Christopher Schroeder believes the new network is tailor-made for reaching wealthy and college-educated women between the ages of 35 and 55--clearly an attractive demographic for health marketers.

"There's nothing now in the health market that compares to what we've just announced," Schroeder said. "And not just size for its own sake, but a network focused on a high-quality audience as well."

Per the agreement, HealthCentral becomes the exclusive seller of pharmaceutical ads across all IAC sites, while IAC will now serve as HealthCentral's major ad provider for over-the-counter drugs and consumer-packaged goods.

Johnson & Johnson, Pfizer, Merck, and AstraZeneca are among the major drug companies that already advertise on HealthCentral.

In January, IAC said it acquired a "significant minority stake" in HealthCentral. The entire investment, which was valued at about $50 million, included additional funds from previous investors Sequoia Capital, Carlyle Group and Polaris Venture Partners.

In addition to the minority stake, Diller and Peter Horan, CEO of the company's Media & Advertising sector, joined HealthCentral's Board of Directors.

"We realized there was a lot of common ground with the audiences between HealthCentral and IAC's core properties and an opportunity to expand the size of campaigns we were selling by partnering," Horan said. He noted, for example, that Evite's user base skews toward women ages 25 to 49, matching up well with HealthCentral's key demographic.

Horan added that the new ad network is the first joint effort in what will be increasing cooperation between the two companies. "The next step is content sharing," he said, referring to the possibility of HealthCentral developing a pediatric health newsletter aimed at mothers in partnership with Evite. He also envisions greater integration of HealthCentral content into Ask.com's Smart Answers feature, which provides informational links above the main results returned on searches.

HealthCentral's myriad rivals include WebMD--which is presently averaging about 40 million monthly users, according to Nielsen//NetRatings--RevolutionHealth, iVillage Health, and About.com Health.

With no other health site seriously threatening WebMD's dominance, Schroeder said the No.2 spot in the category is up for grabs. "I suspect that this deal will help put us in a very strong position, but we still have to earn it," he said.

The joint-venture between IAC and HealthCentral comes on the heels of a ruling by a Delaware Chancery Court judge allowing Diller to split IAC into five companies, spoiling an effort by John Malone--chairman of Liberty Media, the majority voting stockholder in IAC--to prevent the move and send Diller packing.